[2/3] wsdapi/tests: Add tests for Register/UnRegisterNotificationSink

[Owen Rudge]

Hi Nikolay,

 > Don't know if that's important, but this does not test a case when
 > RegisterNotificationSink is called on already registered sink.

I think I did test this scenario myself and the sink would simply be 
registered twice. I didn't think it was worth adding a test for it, but 
I can do so if you think it would be beneficial.

Thanks,

-- 
Owen Rudge
http://www.owenrudge.net/